• Biofloc is a clumpy assemblage of microorganisms (bacteria, microalgae, cyanobacteria, fungi, protozoans, micro-zooplanktons, etc.) with dead particulate organic matter suspended in the water column of an intensively aerated or agitated aquaculture system.
• Exopolysaccharides secreted by bacteria, and certain kind of microalgal species helps in the formation of biofloc. These biopolymers act as adhesives to aggregate the dispersed cells of bacteria, microalgae and other particulate organic matter to form a clumpy mass called bioflocs.
• The Indian vannamei shrimp farming industry has been greatly affected by many disease outbreaks wherein in many cases cultures are lost in a matter of 15 days. This causes huge economic loss as a considerable investment is made in preparation of large grow out ponds. To overcome this, onsite nursery rearing of shrimps from its early postlarvae up to an average size of 1 to 2 g is highly recommended and is an emerging trend.
• Nursery ponds in comparison to grow-out ponds are smaller and can be easily managed for operations faster and at a lower cost even in the case of a disease outbreak.
• In biofloc based nursery rearing, these consortia of bacteria and microalgae clean the unwanted nutrients such as ammonia, nitrite, nitrate, sulphide, etc., which are otherwise toxic to the shrimps by utilising them as a source of nutrient and energy in their metabolic pathway.
• In addition to this cleaning process, biofloc themselves act as a fresh feed (in situ feeding) within the system. This is considered to be a low-cost, sustainable feeding tactic and is receiving popularity in farming of white leg shrimp around the world. Biofloc is found to be a nutritionally rich and balanced with a good amount of protein, minerals and other micronutrients.
• Bioflocs not only act as a feed but also manages the water quality with no any additional cost.
• Nursery rearing of shrimp post larvae (PL) in this biofloc based tanks, ponds or raceways before stocking in the grow-out pond has been observed to increase production by 20 – 30 % and lower the cost of production.
• The PL obtained from the hatchery are reared for a period varying from 20 – 45 days until the PL reaches PL 45 or early juvenile stage of size ranging from 0.3 g to 1.2 g. Stocking of large sized post larvae has been observed to reduce growout period from 20 – 30 days and improve the FCR by 10 – 30 %.
• Stocking larger PL which has been acclimated to the pond conditions has been a strategy to mitigate the issue of diseases like early mortality syndrome (EMS) and white spot syndrome virus (WSSV) infections to some extent.
• As the biofloc based nursery rearing system is preferred over the conventional nursery as it is a zero or minimal water exchange based system.
• The biofloc based nursery rearing technology is based on the adjustment of the C: N ratio in a bacteria-based system.
• The addition of carbon-rich sources in well-aerated ponds stimulates the growth of heterotrophic bacteria which in turn make use of inorganic nitrogen in the form of ammonia and nitrite for generating bacterial protein and thereby create a self-nitrifying system with protein rich flax forming an additional feed to shrimp.
• Postlarvae (PL3) reared in biofloc (107 CFU/ml) based rearing system showed significantly higher growth and survival than in conventional non-biofloc system.

Acute hepatopancreatic necrosis disease (AHPND) earlier known as early mortality syndrome (EMS) or acute hepatopancreatic necrosis syndrome (AHPNS) has been causing significant losses in shrimp farms in China, Vietnam, Malaysia, and Thailand since 2009. The disease affects both black tiger shrimp and Pacific white shrimp and is characterized by mass mortalities during the first 20-30 days of stocking.

Research by the University of Arizona has identified that the disease is caused by the bacterial agent Vibrio parahaemolyticus, which is transmitted orally and colonizes the shrimp’s gastrointestinal tract. This then produces a toxin that causes tissue destruction and dysfunction of the shrimp digestive organ known as the hepatopancreas.

Causes

The disease is caused by a unique strain of Vibrio parahaemolyticus, which can produce toxins responsible for the primary pathology in affected shrimp. Other non-Parahaemolyticus strains such as V. campbellii, V. harveyi, V. owensii, and V. punensis are also found to contain the same toxic genes and may also cause the disease.

Symptoms

• Erratic swimming or swimming near the bottom of the pond
• Reduced growth
• Pale, shrunken or atrophied hepatopancreas
• Whitening of the hepatopancreas
• Reduction in size of hepatopancreas
• Soft texture of the exoskeleton
• Dark spots or streaks on the hepatopancreas
• Hardening of hepatopancreas

Management

• There is no quick fix for EMS/AHPND – once a farm is infected a carefully balanced management plan is required.
• In a worst-case scenario, farmers should be prepared to harvest all ponds at short notice.
• There must implement strict biosecurity measures and a thorough disinfection phase to manage the disease and avoid future outbreaks.
• PL needs to be derived from AHPND-free broodstock. The general health of PL should be checked before stocking, including in stress tests.
• We can manage EMS/AHPND by preventing its further spread and providing better conditions to increase shrimp resistance to it.